DATACMNS-1556 - Deeply inspect query results deeply for the need of query post-processing.

We now inspect returned Collections whether we need to convert these at all. If not, we skip conversion.

Also, allocate GenericConversionService only once for memory-efficiency.

src/main/java/org/springframework/data/repository/core/support/QueryExecutionResultHandler.java

@@ -17,6 +17,8 @@ package org.springframework.data.repository.core.support;
 
 import java.lang.reflect.Method;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
 
@@ -24,11 +26,11 @@ import org.springframework.core.CollectionFactory;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
-import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.core.convert.support.GenericConversionService;
 import org.springframework.data.repository.util.NullableWrapper;
 import org.springframework.data.repository.util.QueryExecutionConverters;
 import org.springframework.data.repository.util.ReactiveWrapperConverters;
+import org.springframework.data.util.Streamable;
 import org.springframework.lang.Nullable;
 
 /**
@@ -44,15 +46,15 @@ class QueryExecutionResultHandler {
 
 	private final GenericConversionService conversionService;
 
+	private final Object mutex = new Object();
+
+	// concurrent access guarded by mutex.
+	private Map<Method, ReturnTypeDescriptor> descriptorCache = Collections.emptyMap();
+
 	/**
 	 * Creates a new {@link QueryExecutionResultHandler}.
 	 */
-	public QueryExecutionResultHandler() {
-
-		GenericConversionService conversionService = new DefaultConversionService();
-		QueryExecutionConverters.registerConvertersIn(conversionService);
-		conversionService.removeConvertible(Object.class, Object.class);
-
+	public QueryExecutionResultHandler(GenericConversionService conversionService) {
 		this.conversionService = conversionService;
 	}
 
@@ -70,10 +72,37 @@ class QueryExecutionResultHandler {
 			return result;
 		}
 
-		MethodParameter parameter = new MethodParameter(method, -1);
+		ReturnTypeDescriptor descriptor = getOrCreateReturnTypeDescriptor(method);
+
+		return postProcessInvocationResult(result, 0, descriptor);
+	}
+
+	private ReturnTypeDescriptor getOrCreateReturnTypeDescriptor(Method method) {
+
+		Map<Method, ReturnTypeDescriptor> descriptorCache = this.descriptorCache;
+		ReturnTypeDescriptor descriptor = descriptorCache.get(method);
+
+		if (descriptor == null) {
 
-		return postProcessInvocationResult(result, 0, parameter);
+			descriptor = ReturnTypeDescriptor.of(method);
 
+			Map<Method, ReturnTypeDescriptor> updatedDescriptorCache;
+
+			if (descriptorCache.isEmpty()) {
+				updatedDescriptorCache = Collections.singletonMap(method, descriptor);
+			} else {
+				updatedDescriptorCache = new HashMap<>(descriptorCache.size() + 1, 1);
+				updatedDescriptorCache.putAll(descriptorCache);
+				updatedDescriptorCache.put(method, descriptor);
+
+			}
+
+			synchronized (mutex) {
+				this.descriptorCache = updatedDescriptorCache;
+			}
+		}
+
+		return descriptor;
 	}
 
 	/**
@@ -81,13 +110,13 @@ class QueryExecutionResultHandler {
 	 *
 	 * @param result can be {@literal null}.
 	 * @param nestingLevel
-	 * @param parameter must not be {@literal null}.
+	 * @param descriptor must not be {@literal null}.
 	 * @return
 	 */
 	@Nullable
-	Object postProcessInvocationResult(@Nullable Object result, int nestingLevel, MethodParameter parameter) {
+	Object postProcessInvocationResult(@Nullable Object result, int nestingLevel, ReturnTypeDescriptor descriptor) {
 
-		TypeDescriptor returnTypeDescriptor = TypeDescriptor.nested(parameter, nestingLevel);
+		TypeDescriptor returnTypeDescriptor = descriptor.getReturnTypeDescriptor(nestingLevel);
 
 		if (returnTypeDescriptor == null) {
 			return result;
@@ -100,7 +129,7 @@ class QueryExecutionResultHandler {
 		if (QueryExecutionConverters.supports(expectedReturnType)) {
 
 			// For a wrapper type, try nested resolution first
-			result = postProcessInvocationResult(result, nestingLevel + 1, parameter);
+			result = postProcessInvocationResult(result, nestingLevel + 1, descriptor);
 
 			if (conversionRequired(WRAPPER_TYPE, returnTypeDescriptor)) {
 				return conversionService.convert(new NullableWrapper(result), returnTypeDescriptor);
@@ -122,7 +151,18 @@ class QueryExecutionResultHandler {
 				return ReactiveWrapperConverters.toWrapper(result, expectedReturnType);
 			}
 
-			return conversionService.canConvert(TypeDescriptor.forObject(result), returnTypeDescriptor)
+			if (result instanceof Collection<?>) {
+
+				TypeDescriptor elementDescriptor = descriptor.getReturnTypeDescriptor(nestingLevel + 1);
+				boolean requiresConversion = requiresConversion((Collection<?>) result, expectedReturnType, elementDescriptor);
+
+				if (!requiresConversion) {
+					return result;
+				}
+			}
+
+			TypeDescriptor resultDescriptor = TypeDescriptor.forObject(result);
+			return conversionService.canConvert(resultDescriptor, returnTypeDescriptor)
 					? conversionService.convert(result, returnTypeDescriptor)
 					: result;
 		}
@@ -130,6 +170,29 @@ class QueryExecutionResultHandler {
 		return Map.class.equals(expectedReturnType) //
 				? CollectionFactory.createMap(expectedReturnType, 0) //
 				: null;
+
+	}
+	private boolean requiresConversion(Collection<?> collection, Class<?> expectedReturnType,
+			@Nullable TypeDescriptor elementDescriptor) {
+
+		if (Streamable.class.isAssignableFrom(expectedReturnType) || !expectedReturnType.isInstance(collection)) {
+			return true;
+		}
+
+		if (elementDescriptor == null || !Iterable.class.isAssignableFrom(expectedReturnType)) {
+			return false;
+		}
+
+		Class<?> type = elementDescriptor.getType();
+
+		for (Object o : collection) {
+
+			if (!type.isInstance(o)) {
+				return true;
+			}
+		}
+
+		return false;
 	}
 
 	/**
@@ -178,4 +241,54 @@ class QueryExecutionResultHandler {
 				|| source == null //
 				|| Collection.class.isInstance(source);
 	}
+
+	/**
+	 * Value object capturing {@link MethodParameter} and {@link TypeDescriptor}s for top and nested levels.
+	 */
+	static class ReturnTypeDescriptor {
+
+		private final MethodParameter methodParameter;
+		private final TypeDescriptor typeDescriptor;
+		private final @Nullable TypeDescriptor nestedTypeDescriptor;
+
+		private ReturnTypeDescriptor(Method method) {
+			this.methodParameter = new MethodParameter(method, -1);
+			this.typeDescriptor = TypeDescriptor.nested(this.methodParameter, 0);
+			this.nestedTypeDescriptor = TypeDescriptor.nested(this.methodParameter, 1);
+		}
+
+		/**
+		 * Create a {@link ReturnTypeDescriptor} from a {@link Method}.
+		 *
+		 * @param method
+		 * @return
+		 */
+		public static ReturnTypeDescriptor of(Method method) {
+			return new ReturnTypeDescriptor(method);
+		}
+
+		/**
+		 * Return the {@link TypeDescriptor} for a nested type declared within the method parameter described by
+		 * {@code nestingLevel} .
+		 *
+		 * @param nestingLevel the nesting level. {@code 0} is the first level, {@code 1} the next inner one.
+		 * @return the {@link TypeDescriptor} or {@literal null} if it could not be obtained.
+		 * @see TypeDescriptor#nested(MethodParameter, int)
+		 */
+		@Nullable
+		public TypeDescriptor getReturnTypeDescriptor(int nestingLevel) {
+
+			// optimizing for nesting level 0 and 1 (Optional<T>, List<T>)
+			// nesting level 2 (Optional<List<T>>) uses the slow path.
+
+			switch (nestingLevel) {
+				case 0:
+					return typeDescriptor;
+				case 1:
+					return nestedTypeDescriptor;
+				default:
+					return TypeDescriptor.nested(this.methodParameter, nestingLevel);
+			}
+		}
+	}
 }

src/main/java/org/springframework/data/repository/core/support/RepositoryFactorySupport.java

@@ -44,6 +44,8 @@ import org.springframework.beans.factory.BeanClassLoaderAware;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.core.convert.support.GenericConversionService;
 import org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor;
 import org.springframework.data.projection.ProjectionFactory;
 import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
@@ -114,6 +116,13 @@ public abstract class RepositoryFactorySupport implements BeanClassLoaderAware,
 		return o;
 	};
 
+	final static GenericConversionService CONVERSION_SERVICE = new DefaultConversionService();
+
+	static {
+		QueryExecutionConverters.registerConvertersIn(CONVERSION_SERVICE);
+		CONVERSION_SERVICE.removeConvertible(Object.class, Object.class);
+	}
+
 	private final Map<RepositoryInformationCacheKey, RepositoryInformation> repositoryInformationCache;
 	private final List<RepositoryProxyPostProcessor> postProcessors;
 
@@ -534,7 +543,7 @@ public abstract class RepositoryFactorySupport implements BeanClassLoaderAware,
 		public QueryExecutorMethodInterceptor(RepositoryInformation repositoryInformation,
 				ProjectionFactory projectionFactory) {
 
-			this.resultHandler = new QueryExecutionResultHandler();
+			this.resultHandler = new QueryExecutionResultHandler(CONVERSION_SERVICE);
 
 			Optional<QueryLookupStrategy> lookupStrategy = getQueryLookupStrategy(queryLookupStrategyKey,
 					RepositoryFactorySupport.this.evaluationContextProvider);

src/main/java/org/springframework/data/repository/util/QueryExecutionConverters.java

@@ -178,7 +178,7 @@ public abstract class QueryExecutionConverters {
 		return SUPPORTS_CACHE.computeIfAbsent(type, key -> {
 
 			for (WrapperType candidate : WRAPPER_TYPES) {
-				if (candidate.getType().isAssignableFrom(type)) {
+				if (candidate.getType().isAssignableFrom(key)) {
 					return true;
 				}
 			}

src/test/java/org/springframework/data/repository/core/support/QueryExecutionResultHandlerUnitTests.java

@@ -54,7 +54,7 @@ import org.springframework.data.util.Streamable;
  */
 public class QueryExecutionResultHandlerUnitTests {
 
-	QueryExecutionResultHandler handler = new QueryExecutionResultHandler();
+	QueryExecutionResultHandler handler = new QueryExecutionResultHandler(RepositoryFactorySupport.CONVERSION_SERVICE);
 
 	@Test // DATACMNS-610
 	public void convertsListsToSet() throws Exception {